Seat belt retractors typically have a spool on which seat belt webbing is wound and from which webbing may be unwound, for example, when a vehicle occupant extends the seat belt webbing around the occupant's body. In response to a vehicle crash condition, the spool is locked against unwinding so that the seat belt webbing will restrain the vehicle occupant in a vehicle seat. To enhance the performance of seat belt retractors over a range of occupant sizes or weights and over a range of vehicle crash conditions, pretensioners have been added to seat belt retractors. Similarly, torsion bars have been added to seat belt retractors.